I hope a few of you caught the surprise northern lights last night. It was definitely not in the forecast but showed up anyway like that friend who’s always welcome. I hit the bed hard at 1 a.m. after the show and managed to get enough sleep to appear human today. But just in case I get drowsy and nod off I wanted to take a few minutes to tell you about tonight’s conjunction between Jupiter and the half-moon.
Close approaches of a planet and the moon — called a conjunction — happen every month. Depending on the moon and planet’s relative positions, sometimes the two bodies are strikingly close and other times less so. Tonight the moon and Jupiter’s distance will vary from about 3.5° (East Coast) to 2° (West Coast) — a fairly close conjunction. The separation shrinks from east to west because the moon moves eastward as it orbits the Earth. Tonight it’s located west of Jupiter and crawls closer as time passes. Since night comes three hours later in San Francisco compared to New York, the moon has had all that time to move east, shrinking its distance from Jupiter.
The moon reaches first-quarter phase tonight, meaning it’s traveled a quarter of the way around Earth in its orbit. At this position, it lies due south at sunset. Light from the sun illuminates half of the moon from our perspective. The other half is still in darkness and will be revealed in the coming nights as the moon waxes to full.
The Midwest and locales further west will get to see the moon precisely 50 percent lit around 11 p.m. Central Time this evening. The terminator, the boundary between the sunlit half of the moon and the half where it is still night, will make a perfectly straight line at that time as if drawn with a ruler. Some hours before 11 p.m., the terminator will appear slightly concave (curving inward), while hours after, it will fatten to convex (curving outward).
I fibbed a little when I said you can see half the moon at first quarter phase. It certainly looks like that on the surface, but in fact you will only see a single quarter of the moon tonight. The shadowed half (still in darkness) is the other “quarter” facing Earth. That makes two quarters. Where are the other two? I bet you can guess. They’re around on the lunar far side!
Directly behind the bright quarter of tonight’s moon and invisible from Earth is the sunlit far side last quarter moon. The far side terminator divides it from the fourth quarter of the moon, still in darkness.
At full moon, the entire front side (near side) is lit by the sun, while the entire far side is in shadow. Phases on the near and far sides are always complimentary.
Hey, want to see Jupiter in daylight in binoculars? Even though the moon is not especially close to the planet you can still use it to spot Jupiter up to an hour before sunset this evening. Point your glass at the moon and focus sharply then look in the lower left hand cover of the view for a little “star” in the blue sky. That’s Jupiter!
Later, when the sky is darker, you can see the moons Callisto and Io on the left side of the planet and Europa and Ganymede on the right.